A polycarbonate resin has been used as a material for molded articles in various industrial fields because the resin is excellent in mechanical property, thermal property, and electrical property. In recent years, the polycarbonate resin has been frequently used in the field of functional products, which utilize the optical property and the like as well as the above-mentioned characteristics of the resin. In association with such expansion of application fields, the variety of performance requested of the polycarbonate resin has also been increasing. Polycarbonate resins having various chemical structures have been proposed for meeting such request because a polycarbonate resin that has been conventionally used, the resin using, for example, 2,2-bis(4-hydroxyphenyl)propane or 1,1-bis(4-hydroxyphenyl)cyclohexane as a raw material, alone may be unable to meet such request sufficiently. However, a requested characteristic specific to each of various applications exists, and hence the development of a polycarbonate resin provided with performance that satisfies such request has been demanded.
An example of such functional products is an organic electrophotographic photoconductor obtained by forming, on a conductive substrate, a photosensitive layer using a polycarbonate resin as a binder resin for a charge generating material or for a charge transporting material.
The organic electrophotographic photoconductor is requested to have predetermined sensitivity, a predetermined electrical characteristic, or a predetermined optical characteristic in accordance with an electrophotographic process to which the organic electrophotographic photoconductor is applied. Operations such as corona charging, toner development, transfer onto paper, and a cleaning treatment are repeatedly conducted on the surface of the photosensitive layer of the electrophotographic photoconductor, and hence electrical and mechanical external forces are applied to the surface every time these operations are conducted. Therefore, the photosensitive layer provided for the surface of the electrophotographic photoconductor is requested to have durability against those external forces in order that the quality of electrophotographic images may be maintained over a long time period.
A polycarbonate resin using, for example, 2,2-bis(4-hydroxyphenyl)propane or 1,1-bis(4-hydroxyphenyl)cyclohexane as a raw material has been conventionally used as a binder resin for an electrophotographic photoconductor, but has not been sufficiently satisfactory in terms of durability. In view of the foregoing, various approaches have been adopted to meet such request. Copolymerization technologies have each been studied so far as an effective technology for improving the wear resistance of a photosensitive layer. Of those, a urethane group introduction method is a technology to which we have paid attention. The urethane group introduction method is a method involving introducing a —O—CO—NH— structure into a polycarbonate copolymer.
A urethane resin is fundamentally formed of a soft segment and a hard segment, and brings together flexibility, toughness, and elasticity by virtue of a balance between the hard segment strongly agglomerated by a strong hydrogen bond between urethane groups as its bonding units and the flexible soft segment. In the urethane resin, the hard segment is formed of a urethane group, and the soft segment is formed of a main chain structure except the urethane group. The inventor of the present invention has considered that high wear resistance is achieved simultaneously with mechanical strength as a feature of a polycarbonate resin by utilizing the foregoing characteristics because of the following reason. When a urethane bond is introduced into the polycarbonate resin, a three-dimensional network structure based on hydrogen bonds is formed, and hence a high-hardness crosslinked surface layer having an extremely high crosslink density is obtained.
However, a urethane-based polycarbonate resin disclosed in Patent Literature 1, which is obtained by copolymerizing an aliphatic diamine monomer and a bisphenol oligomer, has not shown wear resistance at least comparable to one enough to expect the resin to serve as a binder resin for an electrophotographic photoconductor despite the fact that the resin has the features of the hard and soft segments specific to the urethane resin. Although examples of the patent literature have reported aromatic urethane-based polycarbonate resins each obtained by copolymerizing a phenol monoamine monomer and a bisphenol oligomer as well, no significant effect on wear resistance has been observed.
Citation List
Patent Literature
[PTL 1] JP 08-248650 A